The present invention relates generally to gas turbine engines, and, more specifically, to combustors having low exhaust emissions.
In a gas turbine engine, air is compressed in a compressor, mixed with fuel and ignited for generating combustion gases in a combustor, with the gases flowing downstream through one or more turbine stages which extract energy therefrom for powering the compressor and providing useful work. Aircraft gas turbine engines include various configurations having propellers or fans driven by a core engine. The size of the engine, and correspondingly its output power, varies from relatively small turboprop engines to relatively large turbofan engines.
For large commercial turbofan aircraft engines, a significant amount of fuel is burned for propelling the aircraft in flight, and limiting undesirable exhaust emissions therefrom is a significant design factor. At high power operation, low levels of NOx and smoke are desired, with NOx emissions increasing with combustion gas temperature and residence time in the combustor. At engine idle, low CO and hydrocarbon emissions are desired, with longer combustor residence times being desired for reducing these emissions.
In order to accommodate these different requirements for reducing exhaust emissions over the useful operating range of a gas turbine engine, combustion staging is typically provided and includes specifically configured burning zones for reducing exhaust emissions. In one example referred to as a double dome combustor, the combustor is configured with two concentric dome rings spaced radially apart by an annular centerbody, with each of the domes having a plurality of circumferentially spaced apart carburetors mounted therein.
Each carburetor includes a fuel injector discharging fuel into a corresponding air swirler for providing a fuel and air mixture downstream of the respective domes. The air swirlers are stationary, fixed geometry components through which respective portions of compressed air are swirled and mixed with fuel injected from the injectors. Each injector may take any suitable form, with a conventional fuel supply providing fuel thereto at varying flowrates and pressure for varying the output power of the combustor and thereby the output power of the engine.
Fuel staging may be accomplished using the fuel injectors themselves, and fuel staging may also be effected by selectively operating different ones of the several fuel injectors. For example, one of the domes may define a pilot combustion zone, with the other dome defining a main combustion zone, with the fuel injectors for the main zone being off at low power operation of the engine. At high power operation of the engine both the pilot and main zones are supplied with fuel. This combustor configuration allows the fuel/air ratio and distribution to be modulated for reducing the different exhaust emissions from low to high power operation of the engine.
Another conventional embodiment includes a triple dome combustor, which is an extension of the double dome combustor, for yet further reducing the different exhaust emissions over the operating range of the engine. Multi-dome combustors are correspondingly more complex to construct and operate and are typically found in only very large gas turbine engines. The components of a multi-dome combustor are not readily scalable in size for use in relatively small gas turbine engines.
Accordingly, the ability to further modulate fuel and air distribution in a gas turbine combustor for reducing exhaust emissions is desirable. Also desired is the ability to further modulate fuel/air distribution in small combustors, in addition to large combustors.